Sound reproducing apparatus



Nov. 3, 1936. L 5055 2,059,929

SOUND REPRODUCING APPARATUS Filed June 17, 1935 4 Sheets-Sheet l NOV. 3, 1936. BOBB 2,059,929

SOUND REPRODUCING APPARATUS Filed June 17, 1955 4 Sheets-Sheet 2 NOV. 3, 1936. L J B BB 2,059,929

SOUND REPRODUC ING APPARATUS Filed June 17, 1935 4 Sheets-Sheet 5 k [III/11111111111 5 Patented Nov. 3, 1936 UNITED STATES PATENT OFFICE SOUND REPRODUCING APPARATUS Application June 17, 1935, Serial No. 27,097

15 Claims.

This invention relates to systems for propagating or radiating sound energy and has for its principal object the provision of novel means for controlling or regulating the sound energy in such systems. The invention is directed particularly to sound reproducers such as employed in conventional radio receivers wherein certain forms of distortion are occasioned by the radio cabinet and other elements.

A more specific object of the. invention is, therefore, to provide novel means for substantially eliminating the distortion above mentioned.

Under certain circumstances, such as commonly found in a radio loud speaker of conventional form, sound energy propagated from the sound radiator or reproducer may be returned to it with regularity in such a manner as to modify its apparent propagation. This action occurs when a cavity, not necessarily closed, is acoustically coupled to the sound radiator. Within one or more particular frequency ranges, this phenomenon will increase the intensity of the radiated sound. It has been found that a conventional cabinet for a radio loud speaker may exhibit this effect which, for the purpose of this application, will be termed cabinet resonance. In general, the effect is more noticeable when the cabinet is placed near a wall, but it is unnecessary that the cabinet or sides of the cabinet be closed to produce the effect, it being a sufficient condition that propagated energy be returned to the sound radiator for any reason whatsoever.

As is well known, the conventional radio loud speaker comprises a driven diaphragm mounted on a device known as a baffle which serves to prevent sound energy propagated from one side from mixing with the sound energy propagated from the other side. In the case of the radio loud speaker, the cabinet itself may comprise the baffle. In such a system, as the diaphragm moves back and forth, it will generate two sound waves, one on each side of the baffle, which are 180 out of phase with each other; that is, a positive pressure variation on one side of the baffle will correspond to a simultaneous negative pressure variation on the other. In the absence of the baffle, the two sound waves at low frequency will tend to counteract each other and the intensity of sound radiated would be reduced, as is well known.

If now some of the sound energy radiated from one or both sides of the sound radiator with or without a baiile is returned to it in such a way as to be in proper phase relation with, the sound energy being propagated, it will tend to increase the intensity of the radiated sound, causing cabinet resonance. This efiect is particularly undesirable due to the fact that it tends to prolong and augment sounds having a frequency corresponding to the resonant frequency and thus causes distortion. In a conventional radio cabinet, this resonant frequency usually occurs at the lower part of the speech range and thus tends to produce a boomy quality in reproduced speech or music.

A further characteristic of systems in which cabinet resonance occurs, resides in the phenomenon of transient distortion. In certain respect such a system will exhibit the properties of an oscillatory system. For example, if the system is shock excited, it will resonate and propagate sound having the resonant frequency. Likewise, if the system is excited at the resonant frequency, there will be a pronounced transitory delay in the building up and decay of the intensity of sound radiated at such frequency. This delay, which might be termed an acoustic hang-over, is a form of distortion which is particularly undesirable in the reproduction of speech, and cabinet resonance in conventional radio cabinets usually occurs within the lower limits of the speech range. By this invention, there are provided devices which are particularly adapted to prevent this form of distortion, as they absorb sound energy in the resonant frequency range and thus damp out transitory oscillations in a manner similar to the manner in which a resistance added to an electrical 0scillatory circuit will damp out electrical oscillations in it.

Bearing in mind the character of the cabinet resonance phenomenon as above set forth, it will be apparent that if some measure is taken to prevent the building-up process which the sound waves undergo due to the said phenomenon, the deleterious action may be avoided. For example, holes might be provided in the baffle structure and so positioned that-at the resonant frequency, the energy propagated from the opposite sides of the sound radiator will be in proper phase relation to reduce the intensity of sound radiated. This will take place, however, only on a narrow frequency range. Furthermore, under these conditions, the amplitude of motion of the sound-producing diaphragm will be increased due to the fact that the acoustic load on the diaphragm is reduced. This is undesirable as it tends to introduce further distortion due to the larger amplitude of the diaphragm motion. If, however,

some measure is taken by which some of the radiated sound energy or some of the returned sound energy or both may be dissipated, then the cabinet resonance effect may be avoided and at the same time, the acoustic load into which the diaphragm is working will not be materially changed, the net result being that the distortion, due to cabinet resonance is materially reduced and no additional distortion due to increased amplitude of the diaphragm motion is introduced. The absorption of sound energy at the resonant frequency has the additional advantage of imposing a resistance load on the system which tends to reduce any tendency of the system as a whole to oscillate and thus prevents any transient distortion. It will be seen that such a modified system is far superior to a system, such as that above mentioned employing holes in the baffle, since the latter system would have little effect on the transient phenomenon.

The present invention provides means for absorbing sound energy at the resonant frequency, as above mentioned. Such means may consist of tuned diaphragms responsive to particular frequency ranges and which acts simply to absorb sound energy. The energy thus absorbed is dissipated by means of damping elements associated with the diaphragms. These diaphragms may be positioned within the baffle so as to be acted upon by sound radiated from each side of the driven diaphragm or sound radiator. The function of these sound-absorbing diaphragms, however, is not only to reduce the intensity of the sound radi ated but also to absorb some of the sound energy radiated at the resonant frequency. Any sound which might be radiated from these devices is negligibly small and is unnoticeable, particularly in view of the fact that these units are in the vicinity of the sound-reproducing unit or sound radiator which serves to drown out any such effect. Since the sound-absorbing diaphragms are tuned, they will respond only over a limited frequency range and at other frequencies, they will act simply as a part of the inert baffle. It should be noted, also, that the damp ing means associated with the diaphragm will have the effect of broadening the response range thereof and will also reduce the amplitude of response of the diaphragm units. This broadening of the response of the units due to the effect of the damping is an important feature of the invention.

The extent to which the cabinet resonance phenomenon is present in a loud speaker cabinet is a function principally of geometry of the cabinet, as is also the resonant frequency at which the cabinet resonance is effective. In the practice of the present invention to reduce cabinet resonance, several variable factors must be taken into consideration. In the first place, the amount of sound energy absorbed as above described will depend upon the area of the tuned diaphragms, their tuned impedance, and the amount of damping associated with each diaphragm, and the distance between the tuned diaphragms and the driven diaphragm or sound radiator. The frequency range over which the tuned diaphragms are responsive is determined by their mass-tostiffness ratio, the amount of damping, and the amount of additional mass and additional stiffness introduced by the damping. The frequency range may be further controlled by varying the compliance between the edge of the diaphragm and the frame or support to which it is fastened. In general. these tuned d aph agms are analogous to a simple electrical tuned circuit and they act in somewhat the same manner. In this analogy, the mass of a diaphragm corresponds to inductance of a tuned circuit, the stiffness of the diaphragm corresponds to the reciprocal of capacitance, and the damping associated with the diaphragm corresponds to resistance of the said circuit.

The invention may now be considered with reference to certain specific forms illustrated on the accompanying drawings and which will serve to enable a more comprehensive understanding of the principles involved.

In the drawings:

Fig. 1 is a rear face view of a loud speaker mounting board with the invention applied thereto;

Fig. 2 is a perspective view of an open-back loud speaker cabinet, a portion of the cabinet being broken away to disclose the elements with which the invention is concerned;

Fig. 3 is a rear face view of one form of unit which may be employed in accordance with the invention;

Fig. 4 is a sectional view along line 4-4 of Fig. 3;

Fig. 5 is an enlarged fragmentary section of the device;

Fig. 6 is a similar view of a modified form of the device;

Figs. 7 to 9 are fragmentary sectional views illustrating certain embodiments of a portion of the device; and

Fig. 10 is a response curve taken on a cabinet showing the improvement in response due to use of the invention.

Referring to Fig. 1, there is shown a mounting board I, such as is commonly found in a loud speaker cabinet, having a loud speaker or reproducer 2 cooperatively associated with it in the conventional manner. The board I forms part of the loud speaker bafile which also includes at least a portion of the cabinet. The loud speaker comprises the usual diaphragm and suitable actuating means therefor, such as the usual electrodynamic actuating unit. In accordance with the present invention, the mounting board I is provided with one or more openings in addi-- tion to the opening for the loud speaker, and at each of the said additional openings, there is provided a unit such as shown at 3 and 3a. in Fig. 1. These auxiliary units are each equipped with suitable damping means and each is tuned to a particular frequency range. In the device illustrated, the units 3 may be tuned to a certain frequency band, while the unit 3a may be tuned to a lower frequency band. The frequencies to which these units are tuned are those which are pronounced and prolonged by the resonance phenomenon above discussed.

In the specific application of the invention to a radio receiver, as shown in Fig. 2, the speaker bafiie I may comprise the front wall of the cabinet, although it will be understood that the mounting board I may be recessed within the cabinet or may be disposed as desired. The back of the cabinet may be open, as clearly illustrated. The units 3 will be tuned to a frequency or frequencies in the higher range which are objectionably augmented and prolonged by the cabinet resonance effect, while the unit 3a will be tuned similarly to an undesired frequency in the lower frequency range.

As shown more clearly in Figs. 3 and 4, each of the units 3 and 3a mav comprise a supporting pressed by the washers and bolts.

frame 4, a cone diaphragm 5, and a damping element 6. attachment ears I by means of which it may be attached to a wall or baffle as in Figs. 1 and 2. The damping element 6 may take the form of a strip of suitable material such as rubber, duprene, or viscoloid. This strip may be secured at its ends to the frame 4 by means of fibre washers 8 and rivets 9. At a point substantially at the center of the strip, a conical disc I!) formed of light metal may be secured to the strip by a bolt II and a fibrewasher l2. The cone diaphragm 5 may conveniently be formed of paper or like material such as is employed in the conventional loud speaker diaphragm. The apex portion of the cone diaphragm is adhesively secured to the disc ID, as at l3. It will be understood, of course, that the damping means may be mounted in any other suitable manner.

The peripheral edge portion of the cone diaphragm may be formed to provide an integral mounting ring H1 or a separate mounting ring may be employed, as shown at Ma. in Fig. 9. In any event, the diaphragm or the mounting ring therefor may be provided with corrugations, as at l5, so as to floatingly mount the diaphragm with respect to its mounting ring in the manner of the conventional loud speaker diaphragm. As is well known in the art, such mounting renders the diaphragm substantially freely movable as a unit for those frequencies at which the diaphragm is designed to operate. There may be a plurality of corrugations, as shown in Fig. 7, or there may be a single corrugation, as shown in Fig. 8, depending upon the desired characteristics of the unit. The edge portion of the diaphragm, or the mounting ring therefor, may be adhesively secured to the frame 4, as at H5, or otherwise secured thereto.

In any instance, the units 3 and 3a must, of course, be designed to meet the particular requirements, i. e. the units must be designed with respect to the frequencies at which the resonance phenomenon occurs. Several factors will enter into the design of a unit. The material of which the damping element is formed, as well as the dimensions of the free area of the element, will have a direct bearing upon the damping of the unit and are factors which must be considered in the design. The size of the diaphragm and the material of which it is formed are also factors which must be considered in any instance, as they effect the tuning of the unit. The number of corrugations of the diaphragm will also effect the frequency at which the unit is resonant, and the number of corrugations will, therefore, be determined by the design or tuning of the unit to a particular frequency.

In the modification of Fig. 6, the damping element 6a may take the form of a plurality of superposed layers of felt, which may be secured to the frame by washers H and bolts l8. Spacers in the form of sleeves 19 may be provided to limit the degree to which the felt may be coin- A disc or plate 26 may be imbedded in the central portion of the damping element and may be secured to the conical disc lfia by means of the rod 2| extending transversely through the damping element.

The operation of the unit or units provided by this invention is as follows. For sound waves within the frequency range to which it is tuned, the diaphragm of a unit will respond and will absorb sound energy from each side of the The frame 4 may be provided with baffle. Within that frequency range, the motional impedance of the diaphragm is small and largely resistive and it will, therefore, absorb a relatively large amount of sound energy. Therefore, the wave additive process caused by cabinet resonance, as above mentioned, is substantially eliminated. The damping means associa ed with the diaphragm serves to damp out the transitory sound wave oscillations, thus substantially eliminating the distortion caused thereby. For sound waves outside the said frequency range, however, the diaphragm of a unit will possess a very high motional impedance and consequently it will be substantially inert and will function only as a part of the baflle. Since there will be a sufficient number of units of proper design to cover the frequency range or ranges within which cabinet resonance occurs, this undesired phenomenon will be substantially eliminated.

As previously stated, the design of the unit or units to meet the requirements of a particular condition will be governed by certain variable factors already mentioned. The positions of the units will also have some effect upon the results obtained and in this regard it may be stated that in general, the nearer a unit is to the loud speaker or sound radiator, the greater will be the effect of the unit in the reduction of the undesired cabinet resonance effect.

In a specific example which will serve to illustrate the principles of the invention, three of the units were mounted in a conventional radio cabinet having the following inside dimensions: average depth 1 foot. average width 25 inches, and average height 39 inches. at the back and was arranged to contain a conventional radio chassis. The cabinet was equipped with a conventional loud speaker which consisted of an 8% inch diameter cone mounted flush in approximately the center of the front panel of the cabinet. Response curves taken on this device indicated an undesirable increase in response in the frequency range from approximately '70 to 150 cycles per second, indicating that cabinet resonance was present in this frequency range. The curve 22 of Fig. 10 is representative of the response curve thus obtained. Further response curves taken with the cabinet placed near a wall indicated that the resonant frequency range was substantially unchanged but the resonance effect was augmented by the presence of the wall. The distance of the wall from the cabinet seemed to make little or no difference for distances up to six or eight inches.

Following the teachings of the invention, three units similar to those shown in Figs. 3 and 4 were mounted around the loud speaker, one unit comprising a cone 8% inches in diameter was mounted below the loud speaker with its center 11 inches from the center of the loud speaker, while two other units comprising 6 inch diameter cones were mounted above and on each side of the loud speaker with their centers 10 inches from the center of the loud speaker, as shown in Figs. 1 and 2. When properly adjusted, further response curves taken for the cabinet showed that the peak in the response curve in the frequency range of '70 to 150 cycles per second had been substantially flattened out, the maximum reduction being about 10 db. at about c. p. s. curve 23 of Fig. 10 is representative of the response curve thus obtained.

The 10 inch unit was damped as illustrated in Figs. 3, 4, and 5, using a piece of viscoloid inch thick and inch wide, held in place by The cabinet was open The rivets faced with inch washers, the distance between the apex of the cone and each of the mounting rivets being 1 inches. The body of the cone was made of Patterson parchment paper .015 of an inch thick and weighing about 22 pounds per 1,000 square feet. This cone was attached to a rim of kraft paper .006 of an inch thick weighing 60 pounds per ream, the edge hav ing two corrugations as indicated in Fig. 9. Response curves indicated that the resonant frequency of the cone without the damping was 71 c. p. s. and with the viscoloid or damping means attached this resonant frequency was increased to approximately 78 c. p. s. and the overall response curve materially broadened. For the undamped cone the difference in response during an interval of 1 octave was 21 db., whereas for the damped cone it was approximately db.

The two 6 inch diameter units were substantially the same, each comprised a one-piece paper cone, such as shown in Figs. 3 and 8, made of Patterson parchment paper .013 of an inch thick and weighing approximately pounds per 1,000 square feet. A piece of viscoloid A inch by A inch in cross section was used for damping, the distance between the apex of the cone and each fastening means being 1 inch. The resonant frequency for these cones without damping was approximately 91 c. p. s. and 108 c. p. s. with damping. The width of the response curve for these cones was greater than that of the larger cone as indicated by the fact that the difference in response over 1 octave when undamped was 20 db. but only 8 db. with the viscoloid attached. As may be understood the larger unit functions principally during the lower part of the frequency range for which the intensity was to be reduced and the smaller units over the upper part of this region. The response ranges of the units were sufficiently broad so that the overlapping response between resonant points of the damped cones, that is '78 c. p. s. and 108 c. p. s. was sufficiently great to reduce the resonant peak of the cabinet itself, which as previously mentioned occurred at 95 c. p. s.

In addition to the various standard response curves which were taken and two of which have been described hereinabove, oscillographic studies of the transient conditions in the resonant frequency range were made. The decrease in intensity of sound radiated, at frequencies within the cabinet resonance range, due to the presence of the damping units, indicated that the magnitude of the resonance phenomenon had been materially reduced. This fact was corroborated by the studies of the transitory conditions which showed that the magnitude of such transients as were present was materially reduced, and their rate of decay materially augmented. The improvements in the quality of reproduction were, of course, apparent by simple listening tests.

Although the invention is directed particularly to the elimination of cabinet resonance, it will be understood that the principles thereof may be employed in the reduction of undesirable resonance effects regardless of their source. For example, the undesirable effect of resonance of the sound-producing diaphragm or sound radiator may be substantially eliminated by providing a unit of the type herein described tuned to the frequency range within which the resonance effect of the sound radiator occurs. It will be also understood that the invention is not limited by the present disclosure but is susceptible to modifications and changes such as will occur to persons skilled in the art.

I claim:

1. In a system for radiating sound energy, vlbratory means for propagating sound energy, means for actuating said vibratory means, a baffle for said vibratory means, said baffie having openings therein, and frequency selective means 00- operatively associated with the openings in said baffle for selectively absorbing sound energy within at least one predetermined frequency range, said last-named means comprising at least one vibratile diaphragm and damping means associated with said diaphragm.

2. In a system for radiating sound energy, vibratory means for propagating sound energy, means for actuating said vibratory means, a baflle for said vibratory means, said bafile having openings therein, and frequency selective means 00- operatively associated with the openings in said baffle for selectively absorbing sound energy within at least one predetermined frequency range, said last-named means comprising at least one vibratile conical diaphragm and damping means associated with the apex portion of said diaphragm.

3. In a system for radiating sound energy, v1- bratory means for propagating sound energy, means for actuating said vibratory means, a baffle for said vibratory means, said baffle having openings therein, and frequency selective means cooperatively associated with the openings in said bafile for selectively absorbing sound energy within at least one predetermined frequency range, said last-named means comprising at least one unit including a support attached to said baffle, a vibratile conical diaphragm attached at its peripheral edge to said support, and a damping element attached to said support and connected to the apex portion of said diaphragm.

4. In a system for radiating sound energy, vibratory means for propagating sound energy, means for actuating said vibratory means, a baffle for said vibratory means, said baiile having openings therein, and frequency selective means cooperatively associated with the openings in said baffle for selectively absorbing sound energy in accordance with the frequency thereof, said lastnamed means comprising at least two vibratile diaphragms and damping means associated with said diaphragms, said diaphragms selectively absorbing sound energy each at a different fre-- quency.

5. In a system for radiating sound energy,

vibratory means for propagating sound energy,

means for actuating said vibratory means, a baffle for said vibratory means, frequency selective vibratory means acoustically associated with said first vibratory means, and damping means associated with said second vibratory means for absorbing the sound energy selected thereby.

6. In a system for radiating sound energy, vibratory means for propagating sound energy, means for actuating said vibratory means, a batile for said vibratory means, frequency solcctive vibratory means acoustically associated with said first vibratory means, and damping means associated with said second vibratory means for absorbing the sound energy selected thereby, the degree of the damping approaching but being less than critical damping for the frequency to which said second vibratory means is most responsive.

'7. In a system for radiating sound energy, vibratory means for propagating sound energy, means for actuating said vibratory means, a

baffle for said vibratory means, frequency selective tuned diaphragms acoustically associated With said vibratory means, and damping means associated with said diaphragms for absorbing the sound energy selected thereby.

8. In a system for radiating sound energy, vibratory means for propagating sound energy, means for actuating said vibratory means, a battle for said vibratory means, frequency selective tuned conical diaphragms acoustically associated with said vibratory means, and damping means concentrated at the apex of each said diaphragm for absorbing the sound energy selected thereby.

9 In a sound reproducer, a cabinet having a characteristic resonance frequency range, vibratory means acoustically associated with said cabinet for propagating sound energy, a baflie for said vibratory means, frequency selective diaphragms acoustically associated with said cabinet, said diaphragms being tuned to frequencies Within the cabinet resonance frequency range, and damping means associated with said diaphragms for absorbing the sound energy selected thereby.

10. In a sound reproducer, a cabinet having a characteristic resonance frequency range, vibratory means acoustically associated with said cabinet for propagating sound energy, a baffle for said vibratory means, said baiiie having openings therein, and frequency selective means cooperatively associated with the openings in said baffle for selectively absorbing sound energy within said frequency range, said last-named means comprising at least one unit including a support attached to said baflie, a vibratile conical diaphragm attached at its peripheral edge to said support, and a strip of damping material disposed at right angles to the axis of said diaphragm and having its ends attached tosaid support and its central portion attached to the apex of the diaphragm.

11. In a device of the class described, a frequency-selective sound-absorbing unit, comprising a substantially freely-movable floatinglymounted tuned diaphragm, and damping means associated with said diaphragm for absorbing th sound energy selected thereby.

12. In a device of the class described, a frequency-selective sound-absorbing unit, comprising a substantially freely-movable floatinglymounted tuned conical diaphragm, and damping means associated with the apex portion of said diaphragm for absorbing the sound energy selected thereby.

13. In a device of the class described, a frequency-selective sound-absorbing unit, comprising a substantially freely-movable floatinglymounted tuned conical diaphragm having a corrugated edge support, and damping means associated with the apex portion of said diaphragm for absorbing the sound energy selected thereby.

14. In a device of the class described, a frequency-selective sound-absorbing unit, comprising a support, a substantially freely-movable floatingly-mounted tuned conical diaphragm attached at its peripheral edge to said support, and a damping element attached to said support and connected to the apex portion of said diaphragm for absorbing the sound energy selected thereby.

15. In a device of the class described, a frequency-selective sound-absorbing unit, comprising a support, a substantially freely-movable floatingly-mounted tuned conical diaphragm attached at its peripheral edge to said support, and a strip of damping material disposed at right angles to the axis of said diaphragm and having its ends attached to said support and its central portion attached to the apex of the diaphragm for absorbing the sound energy selected thereby.

LLOYD J. BOBB. 

